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 1973 



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 1973 



S N D 



Figure 4. — Monthly mean values for the gonad index and body 

 weight/shell volume index for Mytilus californianus. Circles = 

 control population; dots = outfall population. Vertical lines are 

 the 95'7f confidence limits of the means. Each sample was 12 

 mussels. 



controls, indicating that the control mussels were 

 in better nutritional condition. 



For any given body weight, a larger shell vol- 

 ume will result in a lower body weight/shell vol- 

 ume index. Therefore, the relationship of shell 

 volume to shell length was examined for each of 

 the mussel populations. Over the size range of 

 mussels sampled in the study, this relationship 

 was closely approximated by linear regressions, 

 even though it would probably have been cur- 

 vilinear if much smaller mussels were included in 

 the samples. Shell volumes of outfall M. califor- 

 nianus were proportionally larger than the con- 

 trols over most of the sizes sampled (Figure 5), 

 such that the slope and the intercept of the regres- 

 sion of shell volume on shell length for outfall 

 mussels were significantly different from those of 



70 80 90 

 SHELL LENGTH 



00 

 (mm) 



110 



Figure 5. — Regressions of shell volume on shell length for all 

 Mytilus edulis sampled and for outfall and control samples of Af. 

 californianus. Dashed lines indicate the 95'>'r confidence limits of 

 the mean predicted shell volumes. For each population of mus- 

 sels the sample size, correlation coefficient, and regression equa- 

 tion with the 95'7f confidence limits of the slopes and intercepts in 

 parenthesis was as follows. Mytilus californianus outfall: n = 

 132, r = 0.94, Vol = 1.48( ±0.09)Length - 85. 8( ±8.5l; control: n 

 = 96,r = 0.91, Vol = 1.13( ±0.11lLength - 63.0>±9.0>. Mytilus 

 eduhs outfall: n = 132, r = 0.94, Vol = 1.07(±0.07)Length - 

 47. 2( ±5.2); control: n = 132, r = 0.85, Vol = 1.12( ±0.12lLength 

 - 48,4(±10.8); combined: n = 264, r = 0.88. Vol = 

 1.10(±0-07lLength - 47.9(±6.2). 



the regression for the control mussels (Mests, 

 P<0.05). However, for M. edulis the slopes and the 

 intercepts were not significantly different be- 

 tween regressions of shell volume on shell length 

 for outfall and control populations (^-tests, 

 P>0.05). Therefore, a single, combined regression 

 for both populations of M. edulis was calculated 

 (Figure 5). The difference in the shells of M. 

 californianus may partly account for the apparent 

 differences in the nutritional condition of the out- 

 fall and control populations. 



Discussion 



The reproductive cycle of M. edulis varies with 

 geographical location, but reproductive activity is 

 generally correlated with rising water tempera- 

 tures (Kinne 1970; Seed 1976). Bayne (1975) 

 showed that gametogenesis is regulated by chang- 

 ing temperatures in terms of increasing "day- 

 degrees." In the present study M. edulis from 

 both the outfall and control populations also 



501 



